In an automation.com article, Emerson’s Jesse Sumstad and Neil Widmer explain how accurate measurement of excess oxygen in flue gas can be used to improve combustion control.
These days, industrial facilities operating combustion processes face multiple concerns, including:
- Burner efficiency
- Fuel costs
- Emission restrictions
- Public perception of a company’s carbon footprint and overall environmental responsibility.
All these factors drive the importance of effective combustion control and overall efficiency. The best way to determine combustion efficiency is to analyze what’s going out the stack, starting with residual oxygen. This is not a new idea and many installations have some technology to measure oxygen, but, many operators don’t fully understand what the reading means.
Clearing up this frequent misunderstanding is a key point of our article at automation.com in June, Understanding Oxygen Measurement in Flue Gas Streams. We look at two common measurement technologies, and explain why they can give significantly different measurements in the same application. The reason is, there are effectively two kinds of residual oxygen, and differentiating them requires thinking about combustion itself.
Combustion is a chemical reaction between the fuel and oxygen (O2) and is therefore subject to basic stoichiometric factors. The correct number of O2 molecules must be available to react with the corresponding number of fuel molecules. Air flow imbalance in either direction is problematic. If there is insufficient air (below the stoichiometric requirement, or fuel-rich combustion), unburned fuel goes out the stack. This wastes fuel, creates emissions and hazardous air pollutants. It also creates a potential safety issue should enough fuel subsequently mix with O2 and ignite.
So far, so good, this part is clear. But we know no reaction is 100%, so there will invariably be some unburned fuel. Therefore, the amount of O2 in the stack is a mix of O2 that should have burned to match the amount of fuel, and that which is truly in excess of the stoichiometric requirement.
If there is too much air (above the stoichiometric requirement, resulting in fuel-lean combustion), efficiency is reduced due to energy wasted heating the unnecessary volume of air. This is inescapable to some extent since approximately 80% of air is nitrogen, but excess air is less problematic for efficiency and safer for operation, although nitrogen oxides (NOx) emissions can increase with increasing excess air. For most combustors there is an ideal excess air to achieve good combustion, low emissions and high efficiency.
All that to say, the most important measurement is the true unnecessary volume, but some analyzer technologies can’t tell the difference without a second analyzer to measure the amount of unburned fuel. However, Emerson’s Rosemount™ 6888A In Situ Oxygen Analyzer provides the desired reading because it automatically consumes any O2 in excess of the stoichiometric requirement.
The Rosemount proprietary zirconia sensor has a particular characteristic to avoid this problem. Flue gas, including both leftover O2 and any unburned fuel, flows into the sensor. Given the high temperature and high surface area of the platinum beads, combustion of any unburned fuel and leftover O2 is completed through catalytic-enhanced oxidation. Since all combustion is completed in the sensor, any O2 remaining in the sensor represents excess O2, which is different than the total O2 in the flue gas if unburned fuel is present. Because excess O2 is directly related to excess air, operators can control air flow in real-time to maintain the ideal amount of excess O2.
When operators know the true excess O2 level as delivered by the Rosemount 6888A In Situ Oxygen Analyzer, they can make adjustments to run at the optimum excess O2 percentage. Visit the Combustion Gas Analyzers pages at Emerson.com for more information. You can also connect and interact with other engineers in the Oil & Gas, Chemical, Onsite Utilities, and Power Generation Groups at the Emerson Exchange 365 community.